Definition of Excretory System, and Excretion

In all living beings, the activities of cellular metabolism, which are necessary to maintain life, also generate waste products. These types of waste substances, which not only serve no purpose in the body, but also They can cause health problems and be harmful if they accumulate in the body. They must be eliminated by abroad. In unicellular organisms or those of very small size, cellular transport is sufficient to evacuate the waste, but larger animals need complex organ systems responsible for eliminating this type of waste. products. These organ systems are called the excretory system.

Functions of the Excretory System

The waste produced by cellular metabolism is dumped into the intercellular medium, that is, into the blood or hemolymph. The blood, as it passes through the entire body, also collects the waste that will be eliminated and takes it to the excretory organs.

The excretory system has three main functions: filter blood and eliminate metabolic waste, regular the balance of water and salts and maintain homeostasis of the body.

The body continually produces waste products such as carbon dioxide (CO₂) and nitrogenous compounds, which are a product of protein metabolism, such as ammonia, uric acid and urea, which must be eliminated. The metabolism of proteins and nucleic acids also generates other waste that ends up circulating in the blood.

He liver It is an important organ that has a detoxification function: metabolizes and neutralizes potentially harmful substances that are ingested with food, such as toxins, drugs such as alcohol and medications. This hepatic metabolism generates waste that also ends up in the blood.

All these circulating wastes, if not eliminated, accumulate to dangerous levels and cause poisoning. This is what happens, for example, in people with kidney failure, who need special treatment to purify the blood, called dialysis. This job would normally be done by the kidneys.

Through the gas exchange that occurs on the respiratory surfaces of the respiratory system, the carbon dioxide leaves the body, while the excretory system is responsible for cleaning all the the rest.

Types of excretory systems in animals

Like all organ systems in animals, excretory systems have diversified to adapt to an enormous variety of life forms; but some are recognizable basic structures common to all.

In unicellular animals or those made up of just a few layers of cells, cellular transport is sufficient to perform excretion functions. This is the case of sea sponges, jellyfish, coral polyps and sea anemones.

In larger, and also more complex, animals, there are already organs specialized in excretion.

Structure and function of the excretory systems

In some types of worms, such as flatworms, and other invertebrates there are excretory tubes, called nephridia, which are like cups or funnels open in the body cavity and with an exit to the outside at the other end. The nephridia filter the fluid in that cavity and conduct waste to the outside. Planarians and earthworms have nephridia.

He nephridium type excretory tube is the basic structure of all excretory systems. These tubes have a filter capsule at one end and are open to collector tubes or the outside at the other end.

The tubules are in close contact with the blood or hemolymph, either because the tube itself is immersed in the interstitial fluid or because there are bouquets of blood vessels associated with the tubules, through which blood constantly circulates.

Along the tubes, the original filtrate undergoes exchanges with the surrounding blood, which modifies its composition. Some elements that were originally filtered return to the blood and some waste, which escaped the original filtering, is removed from the blood in these exchanges.

Another of the very important things that happens in these exchanges between the blood and the excretory tubules is the salt homeostasis sodium and potassium in the blood. The blood, or hemolymph, has to have a certain salinity, no more and no less; because an excess or deficiency of these elements can be harmful to health.

During the exchange of blood with the tubules, excess salts pass into the tubule to be excreted with urine. This homeostasis function of the excretory systems prevents the characteristics of the internal environment from changing suddenly and prevents all excess salts from being excreted outside so that they do not cause damage to the body.

That is why the urine of all animals contains, in addition to metabolic waste, salts.

He hydric balance It is the last of the important functions of the excretory system.

To remove all debris, it is required to dissolve them in water, and excrete a watery solution called urine. But, if access to water is a problem, conserving water is vital. The excretory system achieves this by removing water from that solution. When the body needs to conserve water, the urine produced is more concentrated. In mammals this type of urine is darker and has a stronger odor. On the other hand, when water is not a problem, the urine is transparent, because it contains a lot of water.

Types of excretory systems in animals

Although the excretory tubule structure is basic and is present in all animals, the way in which the tubules are organized varies greatly between different types of animals.

Flatworms, annelids, mollusks and some arthropods have the nephridia system with few modifications.

In insects and spiders the tubules are called Malpighian tubules. They function like excretory tubules, filtering hemolymph. The novelty here is that they pour the filtrate into the intestine and therefore, the insects do not urinate, but rather they eliminate the waste filtered by the tubules through the anus, mixed with feces.

The crustaceans, another type of arthropods, have green glands or antennal glands at the base of the antennas, which eliminate debris through pores.

In vertebrates, the tubules are called nephrons and they are grouped into organs called kidneys. There are about 1 million nephrons in each kidney., which greatly increases the filtering capacity compared to nephridia.

In mammals, kidney filtration It runs through the ureters until it reaches the urinary bladder. The bladder expands like an elastic sac to allow more urine to enter. As the bladder fills, the process of urination, or urinating, begins.